Previously known thermosetting powdercoating compositions utilizing unsaturated polyester resins and reactive polymerization initiators have suffered from problems arising from the reactive nature of the ingredients, in particular, problems of premature curing. Generally, all ingredients required for curing the composition have to be contained and uniformly distributed in the powder coating composition when it is applied to an article, which may include a mold, on the surface of which the coating is to be formed. To obtain the requisite uniform distribution of the ingredients, the powder must be thoroughly mixed. Melt mixing, followed by granulation of the mixture, has long been a preferred method of mixing. However, melt mixing has had to be carefully carried out in order to avoid premature curing of thermally labile resins used in the powder coating compositions. As a result of efforts to avoid the problem of premature curing of the powder coating, the mixing was frequently incomplete, resulting in non-uniform distribution of the ingredients and poor quality coatings.
Three temperatures are important for powder coating compositions and powder components thereof. The first of these is referred to as the glass transition temperature (Tg) or melting point (mp) of the powder. In addition to determining the stability of the powder to fusion, this temperature also contributes to and affects significantly the second important temperature, which is referred to as the melt process temperature. Generally, the melt process temperature should be sufficiently above the Tg of the mixture so that the viscosity will be optimal to achieve good mixing. The melt process temperature, in turn, sets limits upon the third important temperature for thermosetting powder coating compositions, which is the cure temperature. The cure temperature should be sufficiently above the melt processing temperature so that cure does not begin during the melt processing or mixing step of the process.
Thus, the three important temperatures are intimately interrelated. A reasonable temperature increment should be maintained between each of these temperatures, but particularly between the melt processing temperature and the cure temperature. As a result of these relationships between the three important temperatures, attempts to reduce the cure temperature generally result in reducing the increments between the three temperatures or in forcing the lowering of the other two temperatures. Of course, reducing the temperature increment between the melt process temperature and the cure temperature is not desirable because the mixture may begin to cure while it is being mixed. Lowering of each of the three temperatures in order to maintain the temperature increments is also not desirable because a drop in the glass transition or melt temperature of the mixture results in a powder with a decreased resistance to fusion, sintering or blocking at ambient temperatures encountered in the normal handling of powder coating compositions.
U.S. Pat. No. 4,075,261, issued Feb. 21, 1978 to Fujiyoshi et al. ("Fujiyoshi") discloses a powder coating composition comprising (1) an unsaturated polyester which is prepared by reacting terephthalic acid or its lower alkyl ester and an .alpha.-olefinic dicarboxylic acid and optionally an organic acid having at least three carboxy groups in the molecule or its functional derivative, and an aliphatic or alicyclic glycol; and (2) an organic peroxide and a molecular crosslinking or bridging agent. Fujiyoshi teaches mixing the composition in a conventional manner, i.e., homogeneously compounding in a mixer, roll mill or kneader, followed by grinding to the desired small particle size.
U.S. Pat. No. 4,873,274, issued Oct. 10, 1989 to Cummings et al. ("Cummings") discloses an unsaturated polyester resin coating powder composition consisting essentially of at least one unsaturated polyester resin; at least one copolymerizable second resin; and an initiator which comprises 1,1'-di(t-butylperoxy)-3,3,5-trimethylcyclohexane, or which contains a small proportion of a fast initiator and a relatively large proportion of a slow initiator.
U.S. Pat. No. 5,593,730, issued Jan. 14, 1997 to Satgurunathan et al. ("Satgurunathan") discloses a powder coating composition which comprises a hybrid polymer system in which granular form of a polymer A formable from monomer(s) by aqueous suspension polymerization and a polymer B, different to A, which polymer system has been obtained using a process which comprises dissolving polymer B in monomeric material to be used in the formation of polymer A, and polymer A and polymer B bear functional groups for imparting curability to the hybrid polymer system.
The present invention solves the problem of uniformly and completely mixing together the components of a thermosetting powder coating composition prior to actual use of the powder to form a coating while avoiding premature curing of the composition.